1. Field of the Invention
The present invention generally relates to an optical 3D measuring apparatus and a chamber volume correcting method for a cylinder head of an engine, and more particularly to a chamber volume measuring apparatus and chamber volume correcting method using an optical 3D measuring technique.
2. Description of the Related Art
Generally, a cylinder head of an internal combustion engine is produced using a die molding technique. A cylinder head produced by the die molding technique has a problem in that the height of the cylinder head and the height of the valve seat surface may vary because of variation in molding condition or aging of the mold. If these dimensions vary, the performance of the engine may be effected. In order to eliminate such a problem, the cylinder head is machined after it is molded so that the variation in the chamber volume is uniform for each cylinder head. The machining is performed according to the result of measurement of the chamber volume.
Japanese Laid-Open Patent Application No.1-210224 discloses a method in which machining of the cylinder head is controlled by the result of measurement of the chamber volume. In this method, the chamber volume is measured according to a pressure difference between a pressure built up in the chamber and a pressure in a master tank by introducing a pressurized air into the sealed chamber. A volume V to be reduced for the chamber is calculated using the pressure difference. A depth to be machined is obtained by dividing V by the average horizontal section area S. However, in this method, the horizontal section area S changes as the machining progresses, an exact chamber volume after the machining cannot be obtained beforehand.
Accordingly, in the conventional method, the chamber volume must be measured after the machining is completed so as to determine whether or not the chamber volume falls between the allowable upper limit V.sub.max and lower limit V.sub.min. This increases the number of manufacturing processes.
Additionally, although the chamber volume can be measured by the conventional method, a contour of a vertical section of the chamber cannot be obtained. Therefore, it is not possible to determine whether or not the molding die of the cylinder head has a defect in shape due to the aging or deformation from some reason.
Japanese Laid-Open Patent Application No.62-220803 discloses an optical 3D measuring apparatus used for measuring a chamber volume. This apparatus uses a slit beam. The slit beam is projected and scanned over an object to be measured. An optical cutting line formed on the object by the slit beam is taken by a video camera so that 3D coordinate information is obtained for a contour of the object to be measured.
FIG. 1 is an illustration showing a structure of the above-mentioned 3D measuring apparatus. In the 3D measuring apparatus, a motor controller 1 generates clock pulses when a volume calculating system 3 supplies a measurement start signal to the motor controller 1 and an optical sensor controller 2. A motor 4 rotates at a constant speed in synchronization with the clock pulses. A supporting member of an optical sensor 5 is moved by means of the motor 4 in a predetermined direction (Y direction) relative to an object 7 to be measured.
The optical sensor 5 projects a slit beam, when it is supplied a clock pulse from the optical sensor controller 2 at every predetermined period, onto the object 7 which slit beam extends in the X direction perpendicular to the Y direction. The optical sensor 5 receives a reflected beam of the slit beam by a video camera, and image information is supplied to the optical sensor controller 2.
The optical sensor controller 2 generates clock pulses at every predetermined period from the time the measurement start signal is received. Coordinate information obtained by the image taken by the video camera provided in the optical sensor 5 is supplied to the volume calculating system 3. The volume calculating system 3 calculates a cross sectional area of the object 7 according to position information of the slit beam along the Y coordinate and the coordinate information of each point on the optical cutting line formed on the object 7.
In the above-mentioned optical 3D measuring apparatus, the motor controller 1 and the optical sensor controller 2 independently generate clock pulses, and thus an operation for the travel of the optical sensor 5 and a measuring operation performed by the optical sensor 5 are performed based on the different clock pulses. Accordingly, there may occur an offset of phase between the two clock pulses. FIG. 2 is a graph showing a relationship between the clock pulses generated by the optical sensor controller 2 and a travel distance of the optical sensor 5 along the axis. Solid lines, shown in FIG. 2-(A), rising from the time coordinate t show clock pulses of the start time of the optical sensor controller 2, and dashed lines show the pulses following the respective clock pulses. FIG. 2-(B) shows a travel distance of the optical sensor 5 from the time when the motor 4 is started. If there is an offset of phase between the two pulse trains generated by the motor controller 1 and the optical sensor controller 2, the start time of the operation of the motor 4 does not correspond to the start time of the measuring operation performed by the optical sensor. That is, if there is an offset of phase, the actual measuring operation starts, as shown in FIG. 2, at one pulse after the pulse at which the measuring operation is to be started, and thus a measuring error may be generated.
Additionally, even if there is no offset of phase between the two clock pulses, the frequencies of the two clock pulses are not always equal to each other due to a tolerance. In such a case, the measured volume may be different from the real volume since the travel distance of the optical sensor 5 does not correspond to the distance which the optical sensor is to be moved. FIG. 3 shows a case where the frequency of the clock pulse generated by the motor controller 1 is slightly less than that of the optical sensor controller 2.
Further, there is another problem, in the above-mentioned conventional optical 3D measuring apparatus, in that a state of the reflected beam received by the optical sensor 5 may vary in accordance with a state of a reflecting surface of the object 7. That is, if there is diffused reflection due to a surface condition of the reflecting surface, a plurality of reflecting beams may enter into the optical sensor 5 which causes generation of noises in the image of the optical cutting line on the object 7. As a result, the image of the optical cutting line has a thicker width, and thus an accurate coordinate point cannot be obtained. Therefore, an accurate measurement cannot be performed.